Gas turbine engines suitable for use in aircraft type applications can presently be placed into one of three broad categories, namely turbojet, turbofan and variable cycle engines. Turbojet engines typically include a rotatable compressor, a combustor and a rotatable high pressure turbine which is connected to the compressor by a shaft to form a spool. In operation, the rotating compressor blades raise the temperature and pressure of air entering the turbojet engine. Fuel is mixed and burned with the air in the combustor. Some of the energy of the rapidly expanding gases exiting the combustor is converted by the turbine into rotation of the shaft which, in turn, rotates the compressor. The gases exit the turbojet engine through a nozzle such that the gases provide a force, or thrust, to the engine.
In contrast, turbofan engines typically include a fan assembly disposed upstream of two or more compressors that are each coupled to one of two or more turbines via coaxial shafts to form two or more coaxially nested spools. For example, a typical turbofan engine includes a low pressure compressor (LCP) coupled to a low pressure turbine (LPT) to form one spool. Downstream of the LPC is a high pressure compressor (HPC) coupled to a high pressure turbine (HPT) disposed upstream of the LPT to form another spool. The combustor is disposed between the HPC and HPT. The spools operate at different pressures and temperatures and rotate at different speeds. Further, in both turbojet and turbofan engines, individual compressors and turbines are subdivided into a number of stages, which are formed of alternating rows of rotor blades and stator vanes.
In a turbofan engine, a portion of the air passing through the fan assembly by passes the engine core and enters an outer air duct while the remaining air enters the LPC. The turbofan engine bypass ratio refers to the ratio of the air flow through the outer duct divided by the air flow though the turbofan engine core. High bypass ratio turbofan engines accelerate a very large mass of air to relatively low exhaust gas velocities. High bypass turbofan engines are better suited for low aircraft speeds where adequate specific thrust can be provided with reduced noise because of the lower exhaust gas velocities. In contrast, low bypass turbofan engines have low air mass flow rates and high exhaust gas velocities. Low bypass turbofan engines and turbojet engines are better suited for high aircraft speeds because of their high exhaust gas velocities.
A third category of aircraft gas turbine engines are known as variable cycle gas turbine engines, which combine the operational characteristics of turbojet or low bypass turbofan engines with the operational characteristics of high bypass turbofan engines. High performance variable cycle gas turbine engines are being designed because of their unique ability to operate efficiently at various thrust settings and flight speeds, both subsonic and supersonic. An important feature of the variable cycle gas turbine engine is its capability of maintaining a substantially constant inlet airflow as its thrust is varied. This feature leads to important performance advantages under less than full power engine settings or at maximum thrust conditions, such as during subsonic cruise.
For example, a variable cycle turbofan engine may include a core engine, first and second fans, and first and second fan bypass ducts. Both fans may have variable pitch inlet guide vanes and variable pitch stator vanes. By varying the pitch of the vanes in the first and second fans, the engine may operate in high bypass and low bypass modes, thereby providing a single engine which is efficient at both low and high aircraft flight speeds respectively. The outer duct walls may be provided with one or more inlets and discharge outlets which can provide additional air to the second fan assembly, depending upon the operational requirements of the engine. Further, U.S. Pat. No. 8,082,727 discloses a variable cycle gas turbine engine with a peripheral duct that receives peripheral inlet air, an auxiliary fan and an auxiliary turbine. The auxiliary turbine is connected to the aft end of the core engine and is configured to receive combustion gases for driving the auxiliary fan that receives air from the peripheral duct. Similarly, U.S. Pat. No. 8,127,528 includes an auxiliary combustor and an auxiliary propulsor or fan disposed in the peripheral duct. Other variable cycle engine designs have been developed and are too numerous to mention here.
Variable cycle engines, with modulated flow areas and geometries, can achieve higher efficiencies than current turbojet or turbofan engines. To optimize variable cycle engine configurations, higher fidelity sensor measurements must be made of the engine operation. The application and use of sensing probes within the hot gas-path environment of the turbine(s) would be highly desirable, and measurements made by such sensing probes could be used to optimally adjust engine parameters for the given speed or determining turbine integrity through active monitoring of airfoil temperatures.
Unfortunately, the maximum operating temperature of currently available sensors are hundreds to thousands of degrees lower than the combustion gas temperatures.